Though present in natural settings at minute quantities, ethylene oxide was first synthesized in a laboratory setting in 1859 by French chemist Charles-Adolphe Wurtz using the so-called “chlorohydrin” process. However, the usefulness of ethylene oxide as an industrial chemical was not fully understood in Wurtz's time; and so industrial production of ethylene oxide using the chlorohydrin process did not begin until the eve of the First World War due at least in part to the rapid increase in demand for ethylene glycol (of which ethylene oxide is an intermediate) as an antifreeze for use in the rapidly growing automobile market. Even then, the chlorohydrin process produced ethylene oxide in relatively small quantities and was highly uneconomical.
The chlorohydrin process was eventually supplanted by another process, the direct catalytic oxidation of ethylene with oxygen, the result of a second breakthrough in ethylene oxide synthesis, discovered in 1931 by another French chemist Thèodore Lefort. Lefort used a solid silver catalyst with a gas phase feed that included ethylene and utilized air as a source of oxygen.
In the eighty years since the development of the direct oxidation method, the production of ethylene oxide has increased so significantly that today it is one of the largest volume products of the chemicals industry, accounting, by some estimates, for as much as half of the total value of organic chemicals produced by heterogeneous oxidation. Worldwide production in the year 2000 was about 15 billion tons. (About two thirds of the ethylene oxide produced is further processed into ethylene glycol, while about ten percent of manufactured ethylene oxide is used directly in applications such as vapor sterilization.)
The growth in the production of ethylene oxide has been accompanied by continued intensive research on ethylene oxide catalysis and processing, which remains a subject of fascination for researchers in both industry and academia. Of particular interest in recent years has been the proper operating and processing parameters for the production of ethylene oxide using so-called “high selectivity catalysts”, that is, Ag-based epoxidation catalysts that are especially efficient at catalyzing the desired product reaction of ethylene and oxygen to ethylene oxide rather than the side reaction of ethylene and oxygen, which produces carbon dioxide byproduct (and water).
However, while high selectivity catalysts have reduced the formation of carbon dioxide byproduct, they may also have increased the production of other undesired byproducts, notably aldehydic impurities such as acetaldehydes and formaldehydes and their associated acids as well as dissociated ions. Acetaldehyde and formaldehyde have long been known as byproducts formed during the operation of ethylene oxide plants. Acetaldehyde is formed as a result of the isomerization of ethylene oxide, while formaldehyde is formed by the reaction of ethylene oxide with oxygen. The associated acids, acetic acid and formic acid, are produced by oxidizing acetaldehyde and formaldehyde, respectively. The presence of aldehydes and their associated acids can negatively affect the UV quality of the ethylene glycol solution and thereby cause degradation of the fiber grade ethylene glycol product. Additionally, the formation of their associated acids (as well as their aldehydic reagents) can decrease the pH to levels sufficiently low to cause corrosion in the plant. These considerations are even more serious in plants that produce fiber grade ethylene glycol. It is additionally important to note that while an impurity like carbon dioxide is produced almost exclusively on the catalyst bed in the EO reactor, acetaldehydes, formaldehydes and their associated acids are produced both on the catalyst and past the catalyst bed.
One possible method of preventing or reducing the corrosion caused by acidic pH levels is to replace the carbon steel components with stainless steel components. While this is highly effective in at least reducing the rate of corrosion if not completely preventing it, using stainless steel components adds some expense and generally cannot be retrofitted into an existing plant. Moreover, this of course does not address the problem of low ethylene glycol product quality resulting from contamination by impurities.
Another possible solution is disclosed in U.S. Pat. No. 4,822,926 in which the reactor product stream is supplied to a quench section (the quench section being disposed inside the EO absorber), and in the quench section the reactor product stream is contacted with a base-containing recirculating aqueous solution in order to neutralize the pH and eliminate some of the organics. The addition of a base, like e.g., sodium hydroxide, does increase the pH (and as a consequence reduces the corrosion in the plant) as well as prevent the formation of some of the organics and aldehydic impurities. But the addition of caustic also frequently causes the decomposition and degradation of the ethylene glycol product, especially for heavier ethylene glycols like triethylene glycol, which often cannot be manufactured to meet minimum quality standards in a process utilizing caustic. Thus, in the end, caustic addition merely exchanges one problem (corrosion and impurity formation) for another (poor product quality).
Other technologies have also been developed in an attempt to reduce the formation of acetaldehyde and formaldehyde and associated impurities. For example, it has long been known that the isomerization of ethylene oxide to various aldehyde species occurs more readily at higher temperatures. This problem can be particularly pronounced as the product effluent leaves the reactor outlet at high temperatures and is largely maintained at such temperatures until entering a heat exchanger in order to cool the gas prior to its delivery to the absorption section.
Thus, techniques and equipment designs have been developed to reduce the temperature of the ethylene-containing product gas as quickly as possible. U.S. Pat. No. 4,376,209 discloses the use of inerts in a cooling zone of the reactor to cool the gas, however, as the patent makes clear, this technique produced mixed results, and possibly actually increases acetaldehyde make as much as suppressing its formation.
Another approach is the integrated reactor-cooler assembly disclosed in U.S. Pat. No. 7,294,317, which is designed to bring about a sharp drop in the temperature of the ethylene-containing gas. However, while the integrated reactor-cooler has shown itself to be extremely successful at reducing the formation of byproducts, it fails to address those impurities that are generated at later processing stages. However, an extensive retrofit and revamp is necessary in order to accommodate the reactor-cooler assembly described in the aforementioned patent.
The use of ion exchange resins requires less reworking for already-existing plants than the integrated reactor-cooler assembly and are highly efficacious at eliminating aldehydic and other impurities from the cycle water. Suitable ion exchange resins are such as those disclosed in U.S. Pat. No. 6,187,973. These ion exchange resins are extremely effective at removing the impurities from the cycle water, without causing the negative consequences mentioned above from caustic treatment. Nonetheless, while ion exchange resins provide excellent performance in removing impurities, difficulties with their use remain. For example, ion exchange resins require frequent regeneration to restore the ability of the resin to capture and adsorb ions and impurities. These regeneration cycles, have to be repeated more frequently as the quantity of byproducts increases. These regeneration cycles considerably complicate the operation of the ethylene oxide process and reduce its efficiency because they require that the spent or exhausted ion exchange bed must be removed from service and a standby bed on-line in its place. This increasing frequency in the regeneration cycle is undesirable because each such cycle requires considerable amounts of demineralized water, and regeneration chemicals; and simultaneously generating waste water that requires treatment and disposal. Given the costs of supplying the demineralized water and chemical, and the costs of treating and disposing of the waste water, there is thus an imperative to reduce the frequency of the regeneration cycles.
Given the foregoing there is a continuing need in the art for techniques to reduce the frequency of regenerating the ion exchange resins.